Technologies in the field of optical disks have been advancing to increase a capacity and density of the same, because most of the information handled by today's personal computers, such as digital moving pictures, demands a large capacity and a high transfer rate. These advances have made optical disks having a several-fold capacity of CD-ROMs (Compact Disk Read Only Memories) almost practically available.
In case of a read only optical disk in which information is recorded in the form of pits, a capacity and density have been increased using techniques of reducing a laser wavelength, a thickness of the disk substrate, etc.
Track density of a conventional optical disk has been selected in such a manner that a resulting track pitch will not be affected by crosstalk. The track density and track pitch are determined by a spot of a laser beam irradiated to the optical disk, and therefore, depends on a wavelength of the laser beam. Likewise, the shortest pit length depends on a diameter of a spot of the laser beam, namely, the wavelength of the laser beam.
Since the recording density in the optical recording depends on a diameter of a spot of irradiated light, techniques to further increase the density have been developed mainly by improving the performance of the optical recording medium or using super resolution technique.
Under these circumstances, an MSR technique for a magneto-optical recording medium and the super resolution technique for read only optical disks have been developed recently as a technique to reduce reproducible data length. The former makes it possible to reproduce information from a magneto-optical recording medium with a higher density by reducing the leakage from the neighboring bits using the fact that a magnetic coupling interaction between the layers of the magneto-optical recording medium varies with temperatures. Whereas the latter makes it possible to reproduce information from a magneto-optical recording medium having a higher density than a conventional technique by using a polymer film, which is opaque at room temperature but starts to transmit laser beams with increasing temperature when irradiated by the same, as a reflection film of the recording pit of the read only disk.
Here, the MSR technique is a technique, with which only the subject data to be read (recording bit) are taken out, so that the reading performance is improved by eliminating the interference of the neighboring data. In the MSR technique, the neighboring data are masked by the reproducing layer, and the masking principle underlies on the magnetic exchange interaction with the recording layer. This is the reason why the technique is so called.
Incidentally, edge recording by combining the above super resolution technique and magnetic field modulation recording is effective to increase density in the magneto-optical recording. However, the reproducing layer is inevitably added to the recording layer, and this film deteriorates the sensitivity of the recording layer to the recording magnetic field. Thus, it has been very difficult to improve the reproductivity of high-density bits while maintaining good sensitivity to the recording magnetic field. Moreover, poor sensitivity to the recording magnetic field causes a problem that the magnetic head consumes a considerable power when recording the information or the reproducing characteristics result unsatisfactorily.